Ray Traced Shadows

This demo implements BVH construction and GPU traversal for rendering hard shadows.

BVH Construction and Layout

BVH is constructed on CPU. The build process is fairly naive, but results in a high quality hierarchy that's fast to traverse. The tree is constructed using a top-down strategy, using a surface area heuristic (SAH) to find optimal split point at every level.

Nodes are laid out in memory using a depth-first traversal order. Child node with the larger surface area is always on the left. This heuristic aims to find an intersected primitive for a ray in a cache-coherent manner.

Each intermediate BVH node is packed into 32 bytes:

struct BVHNode
{
	vec3 bboxMin;
	uint primitiveId;
	vec3 bboxMax;
	uint next;
};

Leaf BVH nodes are packed into 48 bytes:

struct BVHNodeLeaf
{
	vec3 edge0;
	uint padding0;
	vec3 edge1;
	uint next; // node miss pointer
	vec3 vertex;
	uint padding2;
};

BVH Traversal

Hard shadows are implemented by using any-hit BVH traversal for a ray on GPU. A stackless algorithm is used, which relies on the depth-first memory layout of the tree.

The bounding box of each visited intermediate node is tested against a ray. On hit, the next node that must be visited is next in memory. On miss, current node's next pointer is used to skip part of the tree. This either jumps to the current node's right sibling or to the parent's right sibling.

Each intermediate node contains a primitiveId field. If this field is not 0xFFFFFFFF, then current node is reinterpreted as BVHNodeLeaf. Extra data for leaf nodes is stored deinterleaved (at the end of the BVH buffer).

How to build on Windows with Visual Studio 2017

Clone repository

[email protected]:kayru/RayTracedShadows.git
cd RayTracedShadows
git submodule update --init

Generate Visual Studio solution

cd Scripts
cmake-vs2017-vk.cmd

Solution files written to Build directory in the root of the repository.

Acknowledgements

This demo uses similar ideas to what is described in the following work:

• The Perfect BVH, Jacco Bikker, 2016
• Implementing a practical rendering system using GLSL, Toshiya Hachisuka, 2015
• AMD RadeonRays